1. Technical Field
The invention relates to positioning assemblies using electrical linear motors and, more particularly, to such assemblies that accelerate and decelerate a movable working element at a rate approaching or exceeding one G.
2. Discussion of the Prior Art
Improving such positioning assemblies requires breaking the productivity barrier in machining a variety of surfaces and configurations on a given workpiece without dedicated tooling clusters, and to do so with more than one cutting tool that is independently positioned but simultaneously controlled. High-volume prior art machining lines, providing 500,000 machined units per year, or more, of one type, are not economical if marketing demand for such workpiece drops. Such lines may use multiple-spindle turret heads which are costly to change when modifying the line to machine other workpieces, and are limited to only one of either drilling, boring, or milling.
Such problem can be overcome by use of multiple-spindle machines having positioning accelerations many times faster than commercial machining cells. A high degree of flexibility can be achieved by eliminating product-specific worktables, dedicated cutting heads requiring replacement when workpiece changes are made, and utilizing unprecedented speeds not only to complete machining tasks but to change tools or fixtures from an adjacent inventory. Timing for changing to a new product (workpiece) can be substantially reduced, requiring only software control modifications.
However, increased positioning accelerations or decelerations require strong thrust forces that detract from accurate positioning. Maintaining accurate dimensional alignment of a plurality of spindles is complicated by such forces. Reducing the multiple mass while maintaining stiffness in the relatively movable mass remains an associated problem to successfully and accurately position tooling at such unprecedented rates.
The prior art has confined the use of linear motors to positioning small lightweight tools on granite bases or on rigidly-tied tandem axes (axes which are separated transverse to their own extent) (see U.S. Pat. Nos. 4,102,035 and 4,359,814). Little distortion of the supporting structure will be experienced with these devices, allowing use of bearings which contribute little to stiffness, such as air bearings and magnetic loading of mechanical bearings (see U.S. Pat. Nos. 4,392,642; 4,571,799; 4,985,651; and 4,761,876). Air bearings are undesirable because they require special support and guide surfaces that cannot be maintained in a heavy-duty, mass-machining environment for automotive component making, and are undesirable because they are insufficiently dimensionally stiff when deployed to move large tooling at high accelerations or decelerations.
Magnetic loading to increase guidance of a linear motor assembly has been used in conjunction with sliding or roller bearings (see U.S. Pat. Nos. 4,505,464 and 4,985,651). Magnetic loading of bearings does little to enhance stiffness because it is imprecise and weak; magnetic loading is primarily suited to a use that assists in following more closely a guided track and therefore does little to promote stiffness of the linear motor assembly.